Research Interests (Past and Current)

Spacecraft Attitude/Power Control using VSCMGs

I investigated the concept of the Integrated Power/Attitude Control System (IPACS), in which the flywheels are used simultaneously as mechanical batteries and torque actuators. This system is able to store surplus energy into kinetic energy of the flywheels while not disturbing attitude of the spacecraft. This concept has a significant potential to reduce weight and extend the lifetime of a spacecraft by replacing the conventional chemical batteries. I have also worked on adaptive attitude control, singularity analysis and avoidance of CMG/VSCMG, and underactuated attitude control.

ISS CMG Cluster

       Singular Surface of 4-CMG Cluster

Spacecraft Maneuvering for LOS Control with 1 VSCMG


Relative Motion Dynamics and Control of Multiple Spacecraft

I have also studied the relative motion dynamics and control of multiple spacecraft in the general Keplerian orbit. The equations of relative motion are highly coupled and nonlinear, thus controller design is not an easy task. I developed new expressions of the equations of motion and proposed control algorithms which are simpler and also easier to understand.



Adaptive Optics (AO) and Beam Jitter Control

I am currently working with the Spacecraft Research and Design Center, at the US Naval Postgraduate School, to develop advanced control techniques for adaptive optics and beam jitter attenuation. The objective is to improve the performance of optical systems by active compensation of the phase aberration and jitter on optical beams induced by mechanical vibration and atmospheric turbulence. I am developing adaptive filtering techniques to deal with the time-varying nature of typical aberration/jitter and optical system dynamics. The developed controllers experimentally show better performance  in jitter rejection than traditional linear time-invariant controllers. I am also applying this research to the upgrade of control algorithm for the advanced inertia optical reference units (aIRU), which is a state-of-the-art device providing jitter stabilization and line-of-sight knowledge in aerospace optical systems.

Beam Jitter Control Testbed

Target Tracking (PI vs. Adaptive Filter)

Advanced Inertia Reference Unit (aIRU, by ATA)

Adaptive Control of Uncertain Nonlinear Multi-Input Multi-Output Systems

The study of adaptive spacecraft control motivated me to extend my interests to a more generic problem. It is challenging to design an adaptive tracking controller for uncertain nonlinear MIMO systems, and it is successful only for special cases. The innovative adaptive control algorithm that I developed has several unique attributes : 1) it can deal with uncertainties in the input matrix or actuator modeling, and 2) it utilizes physical properties to simplify the algorithm and to reduce the number of uncertainty estimates. The proposed algorithm was successfully used for spacecraft attitude control with misaligned actuators, and it can be applied to other applications as well.


High-Precision Thermal Control in Semiconductor Fabrication

I have worked with Samsung Electronics to develop a state-of-the-art Post Exposure Bake (PEB) unit which uniformly controls wafer temperature during the fabrication process. The PEB is a bottleneck in the current lithography process and  heavily affects the yield of semiconductor chips. My project team also designed and developed a new manufacturing method of a 15-zone ceramic (aluminium nitride) heater using plating and etching processes. The temperature uniformity across a 12''-diameter silicon wafer was controlled within 0.07 deg at a process temperature of 100~300 deg., which was superior to other market-leading products.

Semiconductor Coater/Developer

(PEB inside)

Thermal Uniformity vs. CD Uniformity